This invention relates to a multi-disc brake and in particular, though not exclusively, to a multi-disc brake for use in aircraft. The invention may be applied in, for example, a multi-disc brake of the type comprising friction discs of carbon-carbon composite material or in a brake of the so-called steel type in which steel discs each carry elements of wear-resistant friction material.
An example of a known aircraft disc brake assembly of a multi-steel type is as shown in the accompanying FIGS. 1 and 2 in which FIG. 1 is a sectional elevation of half a brake assembly, in a plane parallel to and coincident with the axis X--X about which the brake assembly is symmetrical, and FIG. 2 is an end elevation of the brake assembly shown in FIG. 1, the left hand half being a section on Y--Y to show the brake assembly with the brake actuators removed.
The disc brake is of the steel type and comprises a non-rotatable annular hub 1 having secured thereto a torque tube 2 by means of bolts 2'. An assembly of annular stator disc members 3,3a,3b are axially slidably keyed, each at its radially inner periphery, to splines 4 on the torque tube 2. A corresponding series of annular rotor disc members 5 are interleaved with the stator members 3 and axially slidably keyed, each at its radially outer periphery 6, to a wheel (not shown) for rotation therewith.
At one, a distal, end of the torque tube 2 an annular flange 7 is secured thereto by bolts 8 for engagement with the axially outermost stator member 3a, to constitute a stop for that stator member.
Twelve brake actuators 9 in the form of hydraulic piston and cylinder assemblies are mounted at equally circumferentially spaced positions at the other end of the torque tube 2 for applying a brake-actuating thrust to the axially innermost stator member 3b. The piston and cylinder assemblies 9 are circumferentially interconnected by a piston housing in the form of an integral flange member 9' which in turn is formed integrally with the aforedescribed annular hub 1.
Thrust transmission means 10 for transmitting thrust from the actuators to the axially innermost stator member 3b comprises first and second concentric and axially spaced metal annuli (11,12) interconnected by a deformable first connecting means 13 in the form of a frusto-conical metal web. The piston 14 of each piston and cylinder assembly 9 is arranged to engage the first annulus 11 to transmit brake applying thrust thereto. A thermal insulation element may be provided between the stator 3b and the pistons 14.
In the aforedescribed construction the circumferentially spaced series of bolts 2' are relied upon to ensure that the torque tube 2 remains secured relative to the annular hub 1 against the separating force arising when the piston and cylinder assemblies 9 are pressurised to apply axial compression to the stack of discs and cause friction to be generated upon relative rotational movement of the rotor and stator discs. To avoid damage and to maintain integrity of operation the bolts 2' are pre-stressed such that under maximum axial load imposed by the piston and cylinder assemblies the bolts ensure that the end of the torque tube 2 remains firmly abutting against the annular hub 1.
It has long been recognised that heat build-up in aircraft brakes of the aforedescribed type can be a major problem and that excessive flow of heat to the wheels, piston housing, hydraulic piston and cylinder assemblies and their associated hydraulic operating systems must be prevented.
Attempts to prevent an excessive flow of heat from the friction discs have included the use of induced air cooling, as described in U.S. Pat. No. 3,301,357 and the use of special load bearing thermal insulators provided at the operating faces of the actuating pistons as described in UK Patent 2124658. While these prior proposals can lead to beneficial affects, the continually increasing thermal demands imposed on modern aircraft brakes make highly desirable the provision of further means for controlling heat transfer from the friction discs.
An object of the present invention is to provide a multi-disc brake having improved means for restricting the flow of heat from the discs to the piston housing of the piston and cylinder assemblies.
In accordance with one of its aspects the present invention provides a multi-disc brake of the kind comprising a torque tube end a piston housing which comprises a plurality of circumferentially spaced piston cylinders each to contain a hydraulically operated piston wherein a thermal isolation element is provided and arranged to restrict heat generated by the friction discs and conducted to the torque tube from flowing from the torque tube to the piston housing.
The present invention provides also a method in which, in a multi-disc brake of the kind comprising a torque tube end a piston housing which comprises a plurality of circumferentially spaced piston cylinders each to contain a hydraulically operated piston, the flow from the torque tube to the piston housing of heat generated by the friction discs and conducted to the torque tube is restricted by provision of an interposed thermal isolation element.
The thermal isolation element preferably comprises a material which behaves in a substantially elastic manner at its service temperature; it may comprise a material having a glass transition temperature (Tg) which is above the service temperature of the element. Therefore, because the material is at a temperature below its Tg, it will have a low hysteresis lose in service. Preferably the element has a modulus of elasticity of more than 2,000 MPa. Preferably the material of the thermal isolation element has a thermal conductivity of less than 2 W/m.k, more preferably less than 0.5 W/m.k. The element may, for example, be a high temperature polymer material such as a polyimide which may be either unfilled or graphite-filled.
The present invention teaches that the thermal isolation element may be provided at the interface between an end of the torque tube and the face of an annular hub to which the torque tube is bolted. The thermal isolation element thereby acts to restrict the flow of heat from the disc stack to the annular hub to which, conventionally, the piston housing is secured or integrally formed.
However, the present invention teaches also that advantageously a piston housing comprising a plurality of circumferentially spaced piston cylinders is not formed integrally with the annular hub and ie separated therefrom by said thermal isolation element interposed therebetween. More preferably the piston housing is arranged to be axially slidable, for example on the torque tube or, more advantageously, on an intermediate support member. Said intermediate support member may be secured directly to the torque tube and/or to the annular hub. It may be formed of a material having a lower thermal conductivity than the material of the torque tube and/or hub.
Preferably the intermediate support member has an end abutment in the form of an axial end stop towards which the piston housing is urged when the hydraulic piston and cylinder assemblies are actuated to apply axial compression to the stack of discs. An element of thermal insulation material preferably is incorporated between the piston housing and said end abutment.
Preferably bias means is provided normally to urge the piston housing away from the end abutment. The bias means may be comprised by one or more metallic springs. One or more fastener devices may be provided to prevent undue axial movement of the piston housing away from the end abutment member under the action of the bias means.
The intermediate support member and/or the torque tube may be provided with spline formations positioned to engage complementary shape formations at an inner periphery of an annular piston housing thereby to restrain rotational movement of the piston housing relative to the hub of the brake. In an arrangement in which the piston housing does not need to react the torque loads arising during operation of the brake, the splines and/or inner periphery of the piston housing may be formed of a material selected primarily on the basis of low thermal conductivity properties in contrast to the strength properties required for the torque tube splines that interengage with the stator discs.
While the intermediate support member may be a separate component mounted on the end of the torque tube, it may alternatively be formed as an integral part of the torque tube. If the piston housing is mounted on an integral torque tube, depending on the space available, the inner periphery of the piston housing may either be slotted and mounted directly on the tube or the inner diameter of the piston housing may be greater than the outside diameter of the splines thereby to ensure that there is no direct contact between the bore of the piston housing and the torque tube.
Typically torque tubes are manufactured from heat-resistant materials such as Ni-Cr-Mo steel or titanium alloy having thermal conductivities (cal/cm-sec-.degree.C.) of 0.09 to 0.2 respectively. However, in practice the use of titanium alloys is limited because of the risk of corrosion if phosphate ester hydraulic fluids are present. If the intermediate support member is a separate component from the main torque tube, the torque tube may be manufactured e.g. from alloy steel and the intermediate support member may be manufactured from a material of a lower thermal conductivity such as austenitic stainless steel or nickel alloy typically having coefficients of thermal conductivity (cal/cm-sec-.degree.C.) of 0.035 to 0.03 respectively.